lens focal length question

[kkamin]

Can someone explain how the lens' local length is measured. I read somewhere it is from the center of the lens, when it is focused on infinity, to the flim or sensor plane.

What is the center of the lens? How do you set the camera on infinity since it seems to be dependent on f/stop--higher f/stops would yield focus to infinity "quicker".

Thanks for reading.

Kkamin

 

[IgsEMT]

Focal length - Wikipedia, the free encyclopedia